Prior to the mid-1970s, red blood cell, platelet and white blood cell differential analyses were typically conducted by manual examination, with a technician viewing blood film slides with the aid of a microscope. Since that time, hematological analysis has been automated, making its use both widespread and commonplace.
While the methodologies for automated analysis vary, most often the enumeration and analysis involves subjecting a diluted sample of whole blood to a lysing reagent which stromatolyzes and eliminates the red blood cell population, and simultaneously modifies the cell membranes of the more prevalent white cell subpopulations. This causes differential shrinkage of the different cell types and enables discrimination and sorting thereof. The size and number of white blood cells in the sample are then detected with the aid of an automated analyzer, by pulling the sample fluid through a sensing zone, which is typically adapted to detect the size (volume) and/or opacity of the blood cells in the sample by electrical or optical differences. The blood cells are counted for a period of time sufficient to gather data for analysis, data points are stored in a memory device, and then analyzed in a processor. The data can then be displayed in the form of a two-dimensional or three-dimensional histogram.
There are various prior art devices for supplying sheath stream and sample fluids to the sensing aperture of a detector. U.S. Pat. No. 3,740,143 shows a system employing peristaltic pumping to supply a series of diluted blood samples to a flow cell for white blood cell differentiation and counting. Peristaltic pumping, which operates by the occlusion or squeezing of the pump tubes, does not provide a sufficiently steady-state flow, and can result in damage to the integrity of the cells, further degrading the accuracy of the device.
U.S. Pat. No. 4,695,431 also shows an apparatus for supplying fluids to a sheath stream flow cell, which employs a single piston pump to inject the sheath fluid into the flow cell with one side of the pump, and simultaneously aspirate the blood sample through the flow cell with the other side of the pump. The piston pump is driven by a drive cylinder operated by controlling the flow of pressurized fluid. By aspirating the blood sample through the flow cell, the suction forces can distort the cells, thus reducing the accuracy of the device. Also, because the single pump is driven by a pressurized cylinder, the fluid quantity cannot be controlled as accurately as may be desired.
For cell or particle analyses of this type, the present inventors have realized that it is advantageous to detect one cell at a time, and accumulate data on thousands of cells. Coincidence, or the simultaneous passage of multiple cells through the sensing zone, can create anomalies or aberrant information. Although this type of information can be partially corrected by using mathematical equations or pulse editing circuits when analyzing the data, important information about the cells may be rejected and thrown away with the sample. This may include information about abnormalities in the sample, since the abnormal cells may give rise to unusual pulses that are rejected in compensating for the passage of multiple cells through the sensing zone. The present inventors have realized that it would be desirable to provide a precisely controlled, steady-state flow of both blood sample and sheath fluids, focused flow, wherein the sample cells are injected through the sensing zone in a substantially single-file relationship relative to each other in order to avoid coincidence and permit accurate detection of cell properties.